tools: ynltool: correct install in Makefile
Use the variable in case user has a custom install binary.
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20251111155214.2760711-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>